As soon as the ponds and wetlands thaw— as early as mid-March—forest preserves across Cook County will be filled with a rejoicing noise. That’s when our first frogs thaw from their winter torpor and begin calling loudly to attract a mate.

The two types of frog you’re most likely to hear first in the spring are the western chorus frog, Pseudacris triseriata, and the spring peeper, Pseudacris crucifer. They’re most vocal from mid-March through mid-April, though the chorus frog often calls later in the year as well.

The call of the chorus frog is frequently compared to the sound your thumb makes when you pull it across the teeth of a comb. The call of the spring peeper is, appropriately enough, a high single “peep.” When many spring peepers call together, they sound a bit like tinkling bells.

Both of these frogs can be found around ponds and wetlands, in open areas and in woods. The best time to hear frogs calling is at dusk and continuing through the night, though they may also be heard during the day.